Implement build cache based on history array

Based on work by KJ Tsanaktsidis

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Signed-off-by: KJ Tsanaktsidis <kjtsanaktsidis@gmail.com>

docs/reference/builder.md

@@ -106,6 +106,13 @@ the `Using cache` message in the console output.
      ---> 7ea8aef582cc
     Successfully built 7ea8aef582cc
 
+Build cache is only used from images that have a local parent chain. This means
+that these images were created by previous builds or the whole chain of images
+was loaded with `docker load`. If you wish to use build cache of a specific
+image you can specify it with `--cache-from` option. Images specified with
+`--cache-from` do not need to have a parent chain and may be pulled from other
+registries.
+
 When you're done with your build, you're ready to look into [*Pushing a
 repository to its registry*](../tutorials/dockerrepos.md#contributing-to-docker-hub).
 

docs/reference/commandline/build.md

@@ -17,6 +17,7 @@ Build an image from a Dockerfile
 
 Options:
       --build-arg value         Set build-time variables (default [])
+      --cache-from value        Images to consider as cache sources (default [])
       --cgroup-parent string    Optional parent cgroup for the container
       --cpu-period int          Limit the CPU CFS (Completely Fair Scheduler) period
       --cpu-quota int           Limit the CPU CFS (Completely Fair Scheduler) quota